In a radio communications system, information data (bits) are translated into symbols in a transmitter. The symbols are then further processed, e.g. expanded by a spreading factor, encoded, etc., before transmission to a receiver. The transmitted signal (symbols) typically propagates through a dispersive medium and arrives at the receiver via multiple paths or channels. In order to efficiently process the received multi-path signals and retrieve the information data carried therein, the channel responses (transfer function) for the different channels (paths) have to be estimated in the receiver. Several channel response estimating techniques are known in the art.
Many of those estimating techniques utilize known control data, so-called pilot data or symbols, included in the transmitted signal. This pilot data is modulated at the transmitter only with known symbols to permit the receiver to obtain a coherent reference that is independent of the unknown information symbols being decoded. The receiver estimates the channel responses based on these received pilot symbols and locally generated or stored pilot reference symbols. The so-obtained channel response estimates are then used for processing the received signals and for retrieving the information data therefrom.
Generally, the more pilot data included in the transmitted signal, the more accurate channel response estimations can be performed in the receiver. However, increasing the pilot data content of the signal will reduce the information data throughput of the transmission since such pilot data content increase comes at the expanse of the amount of (useful) information data that can be carried in the transmitted signal. As a consequence, much effort is invested in improving the channel response estimations without requiring an increase in pilot data content.
Matthew C. Valenti and Brian D. Woerner, “Iterative channel estimation and decoding of pilot symbol assisted turbo codes over flat-fading channels”, IEEE Journal on Selected Areas in Communications, Vol. 19, No. 9, pp. 1 697-1 705, September 2001 describe an iterative channel estimation (ICE) technique. In this ICE, the receiver estimates an initial or preliminary channel response based on pilot symbols included in the received signal. The channel response estimate is multiplied by the received sequence (signal). The result is passed to a demultiplexor, which strips off the pilot symbols. Next, the sequence is passed though a channel deinterleaver and finally to a turbo decoder. This turbo decoder outputs log-likelihood ratio (LLR) estimates of the code symbols. Hard or soft decisions of the code symbols are then determined based on the LLR-values. The symbol estimates are reinterleaved and the (demultiplexed) pilot symbols are reinserted. This whole symbol sequence (including pilot symbols and the tentative information symbol estimates) is fed back to the channel estimator and is used as a known reference sequence for a new (refined) channel estimation using the same received signal. This iterative process with symbol feed backs is repeated several times until accurate channel estimates are obtained.
Although the ICE technique with turbo decoding feedback is rather reliable, this comes at the expanse of high computational complexity and cost and high processing delay. In addition, all received symbols have to be stored and the final deinterleaving and decoding of a received signal have to be put on hold until the iterative process is finished, which might take a non-negligible period of time. As a consequence, a large amount of extra memory is required in a receiver employing ICE. Taken together, these disadvantages make ICE unsuitable for practical implementations in a receiver.
Anna Zhuang and Markku Renfors, “Combined pilot aided and decision directed channel estimation for the RAKE receiver”, Proc. Vehicular Technology Conference 2000, pp. 710-713, September 2000 present an alternative solution to ICE, denoted combined pilot aided and decision directed channel estimation. According to this document, a receiver determines a first channel estimate from reference pilot symbols and received pilot symbols. Data symbols in the received signal are then estimated using this first channel estimate. Decisions are made about the estimated data symbols and these decisions are then used together (as known reference) with the received signal for determining a second channel estimate. A discriminator is utilized for determining which of these two channel estimates to use for the received signal.
Although, this solution has lower complexity than ICE, the probability of error data symbol decision is high and it has a low convergence speed. As a consequence, a much lower performance compared to ICE is obtained.